Direct attachment of capacitors to flip chip dies

ABSTRACT

An integrated circuit package includes a substrate, a flip chip die, and a capacitor. The flip chip die is attached to the substrate via die-to-substrate interconnects. The capacitor is attached to the flip chip die via capacitor-to-die interconnects so that the capacitor occupies a region between the flip chip die and the substrate. Such placement of the capacitor on a flip chip die has the advantage of reducing the distance between the capacitor and its core, thereby reducing unwanted line inductance and series resistance effects. Integrated circuit performance is thereby enhanced.

BACKGROUND

The present invention relates to the electrical, electronic, andcomputer arts, and, more particularly, to integrated circuit packageswith capacitors directly attached to flip chip dies.

Capacitors are frequently paired with microprocessor cores in integratedcircuit (IC) packages to provide decoupling and diminish the effects ofmid-frequency noise. In this case, mid-frequency noise includes thosefrequencies typically between 1 and 500 megahertz (MHz). It is notuncommon, for example, to find packages with processor dies that arevirtually surrounded by arrays of capacitors.

Generally, it is advantageous to place capacitors as close to amicroprocessor core as possible because close placement reduces theamount of line inductance and series resistance between these elements.Simulations suggest, for example, that for every one-millimeterreduction in distance between a capacitor and a microprocessor core,there may be about a seven millivolt reduction in mid-frequency noisedroop. Every ten-millivolt (mV) reduction in droop, in turn, may yieldan additional one-percent improvement in performance as mid-frequencynoise is the limiting performance gain for high end modules that can beaddressed by top surface mount (TSM) capacitors. As signal frequencyincreases, so too do the needs for decoupling capacitors to address thenoise. These decoupling needs are now approaching one gigahertz (GHz) toten terahertz (THz). The Equivalent Series Inductance (ESL) of the pathneeds to typically be less than one picohenries (pH) and the EquivalentSeries Resistance (ESR) needs to typically drop below ten milliohms(me), requiring many parallel connections between the capacitor and theprocessor.

Flip chip dies are used in the mass production of high-end devices suchas microprocessors and graphic chips. In a typical flip chip technology,the underside of a flip chip die is attached to a substrate (sometimescalled an interposer) via a matrix of solder bumps that form solderinterconnects with the substrate. In a flip chip ball grid array(FC-BGA) package, the die/substrate combination may then be connected toa third element (frequently a printed circuit board) via a set of solderballs on the surface of the substrate opposite to that facing the flipchip die.

There is, as a result, a need for new approaches for implementingcapacitors with flip chip dies that allows extremely close placement ofthe capacitors to the flip chip dies.

SUMMARY

Embodiments of the invention describe the attachment of a capacitordirectly to the underside of a flip chip die so that the capacitoroccupies a space between the flip chip die and a substrate. Suchplacement has the advantage of reducing the distance between thecapacitor and its microprocessor core, thereby reducing unwanted lineinductance and series resistance effects.

Aspects of the invention are directed to an integrated circuit packagecomprising a substrate, a flip chip die, and a capacitor. The flip chipdie is attached to the substrate via a plurality of die-to-substrateinterconnects. The capacitor is attached to the flip chip die via aplurality of capacitor-to-die interconnects and occupies a regionbetween the flip chip die and the substrate.

Additional aspects of the invention are directed to a method of formingan integrated circuit package. A capacitor is attached to a flip chipdie via a plurality of capacitor-to-die interconnects. The flip chip dieis attached to a substrate via a plurality of die-to-substrateinterconnects. The capacitor occupies a region between the flip chip dieand the substrate.

Even additional aspects of the invention are directed to an apparatuscomprising a substrate, a flip chip die, and a capacitor. The flip chipdie is attached to the substrate via a plurality of die-to-substrateinterconnects. The capacitor is attached to the flip chip die via aplurality of capacitor-to-die interconnects and occupies a regionbetween the flip chip die and the substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects, and advantages of the presentinvention will become better understood with regard to the followingdescription, appended claims, and accompanying drawings where:

FIG. 1 is a bottom elevational view of a flip chip die and a capacitor,in accordance with an illustrative embodiment of the invention;

FIG. 2A shows a sectional view of the FIG. 1 flip chip die and capacitorattached to a substrate, in accordance with an illustrative embodimentof the invention;

FIG. 2B shows a magnified sectional view of a region of the FIG. 2A flipchip die and capacitor before capacitor attachment;

FIG. 3 shows a flow diagram of an illustrative method for forming theFIG. 2A combination;

FIG. 4A shows a sectional view of a flip chip die and a capacitorattached to a substrate, in accordance with another illustrativeembodiment of the invention;

FIG. 4B shows a magnified sectional view of a region of the FIG. 4A flipchip die and capacitor before capacitor attachment;

FIG. 5A shows a sectional view of a flip chip die and a capacitorattached to a substrate, in accordance with even another illustrativeembodiment of the invention;

FIG. 5B shows a magnified sectional view of a region of the FIG. 5A flipchip die and capacitor just before capacitor attachment; and

FIG. 6 shows a bottom elevational view of multiple capacitors joined toa flip chip die, in accordance with another illustrative embodiment ofthe invention.

DETAILED DESCRIPTION

The present invention will be described with reference to illustrativeembodiments. For this reason, numerous modifications can be made tothese embodiments and the results will still come within the scope ofthe invention. No limitations with respect to the specific embodimentsdescribed herein are intended or should be inferred.

As used herein and in the appended claims, the term “directly” meanswithout any intervening elements.

Embodiments in accordance with aspects of the invention describeflip-chip-based IC packages wherein one or more capacitors are attachedto the bottom of a flip chip die so that the capacitors occupy the spacebetween the flip chip die and its substrate. Attachment of thecapacitors to the flip chip die in this manner has the advantage ofallowing the capacitors to provide decoupling and noise-filteringfunctions while, at the same time, minimizing unwanted line inductancesand series resistances associated with capacitors that are fartherspaced from their microprocessor cores. IC performance is therebyoptimized.

FIG. 1 shows a bottom elevational view of an illustrative flip chip die100 and an illustrative capacitor 200 in accordance with aspects of theinvention. The capacitor 200 is attached to the bottom of the flip chipdie 100 via a plurality of capacitor-to-die interconnects 300, which areshown in phantom because they occupy the space between the capacitor 200and the flip chip die 100. The flip chip die 100 includesdie-to-substrate interconnects 400 for subsequent attachment to asubstrate (not shown). With the flip chip die 100 attached to thesubstrate, the capacitor 200 occupies the space between the flip chipdie 100 and the substrate.

There are several ways of attaching the capacitor 200 to the flip chipdie 100, and subsequently attaching the flip chip die 100 (with thecapacitor 200) to a substrate. One such example is shown in FIGS. 2A and2B, with FIG. 2A showing a sectional view of the flip chip die 100 andthe capacitor 200 attached to a substrate 500 along the A-A′ cleaveplane in FIG. 1, and FIG. 2B showing a magnified sectional view of aregion of the flip chip die 100 and the capacitor 200 before capacitorattachment.

In the embodiment shown in FIGS. 2A and 2B, each of the capacitor-to-dieinterconnects 300 are formed of a combination of a respective capacitorstack 205 and a respective die stack 105 that are brought together andmerge during a reflow process to form a unitary capacitor-to-dieinterconnect 300. In one or more embodiments, for example, the capacitorstacks 205 may each comprise a respective lower capacitor layer 210comprising Ni topped with a respective upper capacitor layer 215comprising Sn before joining (FIG. 2B). Each of the die stacks 105 maycomprise a respective lower die layer 110, also comprising Ni, toppedwith a respective upper die layer 115 comprising Au before joining (FIG.2B).

The die-to-substrate interconnects 400, in turn, attach to andinterconnect die pads 120 on the flip chip die 100 to substrate pads 505on the substrate 500 (FIG. 2A). In one or more embodiments, thedie-to-substrate interconnects 400 may comprise solder bumps formed of,for example, SnBi (e.g., 42 wt % Sn; 58 wt % Bi) or PbSn (e.g., eutectic37 wt % Pb; 63 wt % Sn). The die pads 120 may comprise, as just a fewexamples, Cr—Cr:Cu—Cu—Au; Ti—Cu; Ti:W—Cu; Ti—Ni—:V, Cr—Cr:Cu—Cu; Al—Ni:V—Cu; or Ti:W(N)—Au, in accordance with conventional “under-bumpmetallization” (UBM). The substrate pads 505 may comprise, for instance,Cu, NiAu, NiAuPd, or some combination thereof.

As will be further discussed below, it is contemplated that thecapacitor 200 will be attached to the flip chip die 100 before the flipchip die 100 is attached to the substrate 500 (i.e., before die attach).As a result, it is preferred that the capacitor-to-die interconnects 300not reflow during the thermal processing required for die attach. Statedanother way, the ultimate melting point temperature of thecapacitor-to-die interconnects 300 will preferably be higher than thereflow temperature of the die-to-substrate interconnects 400. Otherwise,the capacitor-to-die interconnects 300 may melt during die attach and becompromised.

Where the upper capacitor layers 215 comprise Sn and the upper dielayers 115 comprise Au, as in the example presented above, thecapacitor-to-die interconnects 300 may be formed by bringing thecapacitor stacks 205 into contact with their corresponding die stacks105 and heating the combination to about 250° C. This will have theeffect of melting the Sn (with a pure Sn melting point temperature ofabout 232° C.) and gettering the Au into the molten Sn to form an SnAubonding layer by what is sometimes called a “moving boundary” effect.Once cooled and solidified, the just-formed SnAu bonding layer will havea melting point temperature of some 280° C. assuming about 20 wt % Sn.This melting point is about 60° C. higher than the reflow temperature of217° C. typically used to reflow SAC (Sn—Ag—Cu) solder bumps. A 50+° C.temperature difference between the melting point temperature of thecapacitor-to-die interconnects 300 and the reflow temperature of thedie-to-substrate interconnects 400 (hereinafter called the “temperaturedelta”) is considered more than sufficient. In fact, where very preciseheating means are utilized for subsequent die attach, substantiallysmaller temperatures deltas (e.g., 10° C. or less) may be entirelyadequate. Such precision may be obtained utilizing conventional methodsand tooling including, for example, some thermal compression bondingmachines.

In another embodiment, as just another example, the capacitor stacks 205and the die stacks 105 may be similarly composed to one another, withthe lower capacitor layers 210 and the lower die layers 110 comprisingCu, and the upper capacitor layers 215 and the upper die layers 115comprising SnAg (e.g., 96.5 wt % Sn; 3.5 wt % Ag). Once broughttogether, reflow can be performed at about 250° C. The resulting SnAgbonding layer will then solidify with a melting point temperature ofabout 220° C. So long as the die attach is performed at temperatureslower than this, there should be no issue with the capacitor-to-dieinterconnects 300 melting or otherwise deforming during the subsequentprocessing. Eutectic SnBi has a melting point temperature of about 139°C., allowing reflows to be performed at under 150° C. if desired, andmaking this choice of solder a good choice in many embodiments of thepresent invention. A temperature delta of greater than 20° C. maythereby be obtained utilizing higher melting point solders such as SnAgfor the capacitor-to-die interconnects 300 and lower melting pointsolders such as Sn—Bi40—Cu0.5—Ni0.03 that also melt at 139° C. for thedie-to-substrate interconnects 400. Other candidate solders for thecapacitor-to-die interconnects 300 may include 95.5Sn3.9Ag0.6Cu and97Sn3Cu, which have melting point temperatures around 220° C. and 230°C., respectively. Nevertheless, this list is not meant to be exhaustiveand any other suitable materials would still come within the scope ofthe invention.

Two candidate capacitor types for the capacitor 200 are silicon deeptrench (Si DT) capacitors and metal-insulator-metal (MIM) capacitors. Acombination of these types of capacitors may also be utilized. Both SiDT and MIM capacitors will already be familiar to one having ordinaryskill in the relevant arts. Si DT capacitors utilize silicon substratesthat house an array of deep trench capacitors patterned into the body ofthe capacitor. MIM capacitors utilize metal plates separated bydielectric materials. In the present embodiment, the illustrativecapacitor 200 is shown with six capacitor-to-die interconnects 300 (FIG.1), but this number is merely by way of illustration. In actualreduction to practice, fewer or greater numbers of capacitor-to-dieinterconnects 300 may be utilized.

Given that the capacitor 200 will occupy the space between the flip chipdie 100 and the substrate 500, the combined height of the capacitor 200and the capacitor-to-die interconnects 300 is preferably smaller thanthe combined height of the die pads 120 and the die-to-substrateinterconnects 400 so as to avoid interference. To accommodate thislimitation, each of the capacitor-to-die interconnects 300 preferablyhas a height smaller than that of each of the die-to-substrateinterconnects 400. In some cases, the capacitor 200 may also have to bethinner than those utilized in other decoupling roles. Capacitors with30-50 micrometer (μm) thicknesses may, for example, be utilized. Bothmodern Si DT capacitors and MIM capacitors may demonstrate capacitancedensities of up to three microfarads per square millimeter (μF/mm²),giving even thinned capacitors adequate capacitances to provide thedesired decoupling and filtering functions.

FIG. 3 shows a flow diagram of an illustrative method 1000 for formingthe combination of the flip chip die 100, the capacitor 200, and thesubstrate 500 in FIGS. 2A and 2B. In presenting this method 1000,however, it should be recognized that while the structures providedherein are entirely novel, many of the individual processing stepsrequired to implement the method may utilize conventional semiconductorfabrication and packaging techniques, as well as conventional tooling.These techniques and tooling will already be familiar to one havingordinary skill in the relevant arts. Moreover, details of the individualprocessing steps used to fabricate semiconductor and to package devicesdescribed herein may be found in a number of publications including, forexample, P. V. Zant, Microchip Fabrication, Sixth Edition: A PracticalGuide to Semiconductor Processing, McGraw Hill Professional, 2013; S.Wolf, Silicon Processing for the VLSI Era, Vol. 4: Deep-SubmicronProcess Technology, Lattice Press, 2003; and H. Geng, SemiconductorManufacturing Handbook, Second Edition, McGraw Hill Education, 2018,each of which are incorporated by reference herein. It is alsoemphasized that the descriptions provided herein are not intended toencompass all of the processing steps that may be required tosuccessfully form a functional device. Rather, certain processing stepsthat are conventionally used in forming packaged integrated circuitdevices are purposefully not described herein for economy ofdescription. However, one skilled in the art will readily recognizethose processing steps omitted from this more generalized description.

In the method 1000, the flip chip die 100 is prepared for joining to thecapacitor in step 1005 while the flip chip 100 is still part of alarger, intact wafer (i.e., at wafer level). The die stacks 105 may beformed on the flip chip die 100 by, for example, coating the wafer inresist, and then exposing and developing the resist to create openingsin the resist over those regions where the die stacks 105 are intended.Electroplating may then be utilized to form the die stacks 105 with thelower and upper die layers 110, 115 in these openings. The resist maythen be stripped.

The die pads 120 and the die-to-substrate interconnects 400 may beformed on the flip chip die 100 by standard solder bumping techniques.In one or more embodiments, for example, a “mushroom” solder bumpingprocess may be utilized. A UBM layer for the die pads 120 may bedeposited by sputtering, and a resist coated and developed on this layerwith openings where the die pads 120 and solder bumps are intended.Solder bumps may then be electroplated onto the UBM layer through theopenings in the resist, and the resist stripped to create mushroomshaped solder features. The UBM layer may then be etched using themushroom-shaped solder bumps as a hard mask to define the die pads 120,and the solder bumps reflowed to create the spherical die-to-substrateinterconnects 400 (i.e., spherical solder bumps) visible in FIG. 2B.

In an alternative process, a “photo stencil” process may be utilized toform the die pads 120 and the die-to-substrate interconnects 400,wherein a UBM layer is deposited by sputtering and patterned into thedie pads 120 by photolithography and etching. A dry film resist may thenbe coated, exposed, and developed on the film stack to create openingsover the die pads 120. Solder past may then be deposited into theopenings, the dry resist stripped, and the solder paste reflowed toagain create the spherical die-to-substrate interconnects 400 (i.e.,spherical solder bumps) visible in FIG. 2B. It is noted, however, thatwhile mushroom- and photo-stencil-based bumping techniques areexplicitly described above, any other suitable techniques would alsofall within the scope of the invention. These alternative techniquesinclude, but are not limited to, stud bumping, solder jetting,evaporation, and electroless bumping.

Step 1010 involves preparing the capacitor 200 for joining to the flipchip die 100. Preparation of the capacitor stacks 205 on the capacitor200 in step 1010 may be by a process similar to that used to prepare thedie stacks 105 on the flip chip die 100 in step 1005. That is, a processflow comprising resist deposition and exposure, electroplating, andresist stripping may be utilized (see above).

With the flip chip die 100 and the capacitor 200 prepared in thismanner, the capacitor 200 may be joined to the flip chip die 100, again,with the flip chip die 100 still part of the wafer. Step 1015 may beperformed before wafer level testing or afterwards, as desired. Toaccomplish the attachment, the capacitor stacks 205 may be brought intocontact with the die stacks 105 and heat applied to the upper die layers115 and the upper capacitor layers 215. The choice of materials for theupper die and capacitor layers 115, 215 will dictate the necessarytemperature. During this process, the die-to-substrate interconnects 400(i.e., solder bumps) may melt and re-solidify. Nevertheless, studieshave shown minimal impact on solder bump integrity even after a solderbump has been reflowed some ten times. Moreover, the spaces between thecomponents is preferably adequate to keep any molten die-to-substrateinterconnects 400 sufficiently apart from the capacitor 200. Thesidewalls of the capacitor 200 are also preferably not wettable.

In step 1020, the wafer containing the flip chip die 100 may be diced tocreate individual flip chip die, including the flip chip die 100. Instep 1025, the substrate 500 is prepared for joining to the flip chipdie 100. In some embodiments, for example, the substrate pads 505 may betopped with a pre-solder layer comprising a solder similar to that usedfor the die-to-substrate interconnects 400 (i.e., the solder bumps). Inother embodiments, where, for example, the substrate pads 505 compriseNiAu or NiPdAu, it may be possible to forego the pre-solder layer. Ifpre-solder is utilized, deposition of the pre-solder may be byelectroplating at the same time the underlying substrate pads 505 areformed. The pre-solder may be reflowed after deposition.

Attachment of the flip chip die 100 to the substrate 500 occurs in step1030. A layer of flux may be deposited on the substrate 500 beforebringing the die-to-substrate interconnects 400 into contact with thesubstrate pads 505 (and any pre-solder) and heating to reflow thedie-to-substrate interconnects 400. Subsequently, the combination inFIG. 2A may be underfilled to strengthen the attachments of the variouscomponents.

In this manner, an IC package is formed comprising a substrate (e.g.,the substrate 500), a flip chip die (e.g., the flip chip die 100), and acapacitor (e.g., the capacitor 200). The flip chip die is attached tothe substrate via a plurality of die-to-substrate interconnects (e.g.,die-to-substrate interconnects 400). The capacitor is attached to theflip chip die via a plurality of capacitor-to-die interconnects (e.g.,capacitor-to-die interconnects 300) and occupies a region between theflip chip die and the substrate (e.g., the capacitor 200 occupies aregion between the flip chip die 100 and the substrate 500 in FIG. 2A).

FIGS. 4A and 4B show a flip chip die 100′ and a capacitor 200′ attachedto a substrate 500′ in accordance with another illustrative embodimentof the invention. FIG. 4A shows a sectional view of the flip chip die100′ and the capacitor 200′ attached to the substrate 500′ along acleave plane like the A-A′ cleave plane in FIG. 1, and FIG. 4B shows amagnified sectional view of a region of the flip chip die 100′ and thecapacitor 200′ just before capacitor attachment.

In the alternative embodiment shown in FIGS. 4A and 4B, the flip chipdie 100′ and the capacitor 200′ utilize different capacitor-to-dieinterconnects 300′ from those presented earlier, while thedie-to-substrate interconnects 400′ and the associated die pads 120′ andsubstrate pads 505′ remain largely identical. More particularly, insteadof using capacitor stacks and die stacks in the manner of the capacitor200 and the flip chip die 100, the capacitor 200′ is attached to theflip chip die 100′ by preparing capacitor metallic features 220 on thecapacitor and die metallic features 125 on the flip chip die 100′, andthen merging these metallic features 125, 220 together to create thecapacitor-to-die interconnects 300′. Reference to FIG. 4B will show thateach of the capacitor metallic features 220 is disposed in a capacitoroxide layer 225 on the capacitor 200′, and each of the die metallicfeatures 125 is disposed in a die oxide layer 130 on the flip chip die100′. If the die and capacitor metallic features 125, 220 comprise Cu,for example, they will merge when brought together and heated to about200° C., creating the capacitor-to-die interconnects 300′. So formed,the capacitor-to-die interconnects 300′ will display melting pointtemperatures over 1,000° C. Thus, the aforementioned temperature deltais quite large using the metallic features 125, 220 for thecapacitor-to-die interconnects 300′.

To achieve the combination shown in FIGS. 4A and 4B, steps 1005-1015 inthe method 1000 are modified from those discussed above, while steps1020-1030 remain largely the same. In one or more embodiments, forexample, formation of both the die metallic features 125 and thecapacitor metallic features 220 in steps 1005 and 1010 may be performedby damascene processes. For the capacitor 200′, the capacitor oxidelayer 225 may be formed on the capacitor 200′, and then trenches etchedinto the capacitor oxide layer 225 where the capacitor metallic features220 are desired utilizing photolithography and anisotropic etching.Subsequently a blanket layer of metal (e.g., Cu) may be deposited on thecapacitor oxide layer 225 and planarized by chemical mechanicalplanarization (CMP) so that the metal only remains in the trenches. Asimilar damascene process may be performed to form the die metallicfeatures 125 in the die oxide layer 130. The die and capacitor metallicfeatures 125, 220 may then brought together and heated to join thecapacitor 200′ to the flip chip die 100′ (still part of a wafer) in step1015.

FIGS. 5A and 5B show a flip chip die 100″ and a capacitor 200″ attachedto a substrate 500″ in accordance with even another illustrativeembodiment of the invention. FIG. 5A shows a sectional view of the flipchip die 100″ and the capacitor 200″ attached to the substrate 500″along a cleave plane like the A-A′ cleave plane in FIG. 1, and FIG. 5Bshows a magnified sectional view of a region of the flip chip die 100″and the capacitor 200″ just before capacitor attachment. Notably, in amanner different from that set forth above, the substrate 500″ utilizesraised substrate studs 510 to interconnect with the flip chip die 100″(FIG. 5A). That is, the substrate 500″ does not include substrate padslike the substrate pads 505, 505′ in FIGS. 2A and 4A.

In steps 1005 and 1010, the flip chip die 100″ and the capacitor 200″are prepared for interconnection by forming die stacks 105″ andcapacitor stacks 205″ similar to the die stacks 105 and the capacitorstacks 205, respectively, set forth above. Die pads 120″ are also formedon the flip chip die 100″. Formation of the die pads 120″ may be by, forexample, depositing a blanket UBM layer and then patterning that layerby photolithography and anisotropic etching. In a departure from theprevious embodiments, no die-to-substrate interconnects (i.e., solderbumps) are formed in step 1005.

Subsequently, steps 1015 (joining the capacitor 200″ to the flip chipdie 100″ (still part of a wafer)) and step 1020 (wafer dicing) areperformed in the manner set forth above. However, in step 1025, thesubstrate is prepared with substrate studs 510 topped with pre-solder,which will ultimately become the die-to-substrate interconnects 400″.The substrate studs 510 and pre-solder on the substrate 500″ may beformed by, for example, coating the substrate 500″ in resist, and thenexposing and developing the resist to create openings in the resist overthose regions where the substrate studs 510 are intended. Electroplatingmay then be utilized to form the substrate studs 510. The resist maythen be removed and the shorting electroless metal (e.g., Cu) layeretched away. Next a permanent solder resist may be applied, and thenexposed and developed to create openings in the solder resist over thesubstrate studs 510. At this point, solder may be either screened on orsolder ball drop used to apply solder in these regions. Instead of usingonly pre-solder, ball standoffs may also be utilized to form thedie-to-substrate interconnects 400″ on the substrate studs 510. In thisway, standoff heights of as much as 100 μm can be achieved.

The substrate studs 510 may comprise, for example, Cu, and thepre-solder layers may comprise any suitable solder, including, forexample, low melting solders such as SnBi-based solder, In-based solder,eutectic PbSn, or various combinations of Pb-free solder, etc. In step1030, die attach may be performed by bringing the pre-solder layers onthe substrate studs 510 into contact with the die pads 120″ on the flipchip die 100″, and heating to reflow the pre-solder to form thedie-to-substrate interconnects 400″. As before, a layer of flux may beapplied before die attach or an atmosphere of formic acid may beutilized in step 1030 to aid with cleaning, promoting wetting, andreducing oxidation during die attach.

While attachment of a single capacitor to the bottom of a flip chip dieis described in each of the embodiments above, multiple capacitors maybe attached to the underside of a given flip chip die when aspects ofthe invention are reduced to practice. FIG. 6 shows a bottom elevationalview of a flip chip die 600 with four capacitors 700 attached to itsunderside before die attach, in accordance with an illustrativeembodiment of the invention. The flip chip die 600 includes a pluralityof die-to-substrate interconnects 800, which, in the present embodiment,are in the form of solder bumps. The ideal location for each capacitoris typically adjacent to the high power core.

It should again be emphasized that the above-described embodiments ofthe invention are intended to be illustrative only. Other embodimentsmay, for example, utilize different materials and configurations fromthose expressly set forth above to achieve embodiments falling withinthe scope of the invention. These numerous alternative embodimentswithin the scope of the appended claims will be apparent to one skilledin the art. For example, while several means of interconnection betweencapacitors, flip chip dies, and substrates are discussed above, otherequally suitable means of interconnection would also fall within thescope of the invention.

All the features disclosed herein may be replaced by alternativefeatures serving the same, equivalent, or similar purposes, unlessexpressly stated otherwise. Thus, unless expressly stated otherwise,each feature disclosed is one example only of a generic series ofequivalent or similar features.

Any element in a claim that does not explicitly state “means for”performing a specified function or “step for” performing a specifiedfunction is not to be interpreted as a “means for” or “step for” clauseas specified in AIA 35 U.S.C. § 112(f). In particular, the use of “stepsof” in the claims herein is not intended to invoke the provisions of AIA35 U.S.C. § 112(f).

The descriptions of the various embodiments of the present inventionhave been presented for purposes of illustration, but are not intendedto be exhaustive or limited to the embodiments disclosed. Manymodifications and variations will be apparent to those of ordinary skillin the art without departing from the scope and spirit of the describedembodiments. The terminology used herein was chosen to best explain theprinciples of the embodiments, the practical application or technicalimprovement over technologies found in the marketplace, or to enableothers of ordinary skill in the art to understand the embodimentsdisclosed herein.

What is claimed is:
 1. A method of forming an integrated circuitpackage, the method comprising the steps of: attaching a capacitor to aflip chip die via a plurality of capacitor-to-die interconnects bybringing a capacitor metallic feature into contact with a die metallicfeature and heating the capacitor metallic feature and the die metallicfeature; attaching the flip chip die to a substrate via a plurality ofdie-to-substrate interconnects connected to a plurality of die pads onthe die by heating the die-to-substrate interconnects; wherein thecapacitor occupies a region between the flip chip die and the substrate;further comprising avoiding interference of the capacitor with thesubstrate by sizing each of the plurality of the capacitor-to-dieinterconnects to have a height smaller than that of each of theplurality of die-to-substrate interconnects; and by sizing a combinedheight of the capacitor and the capacitor-to-die interconnects to besmaller than a combined height of the die pads and the die-to-substrateinterconnects; wherein the capacitor metallic feature is disposed in afirst oxide layer that covers a surface of the capacitor facing the flipchip die and is interposed between the capacitor and the flip chip die;the die metallic feature is disposed in a second oxide layer that isformed in the flip chip die; and surfaces of the die metallic feature,the second oxide layer, and the flip chip die that face the capacitorare coplanar.
 2. The method of claim 1, wherein the step of attachingthe flip chip die to the substrate comprises heating the flip chip dieand the substrate to a maximum temperature below a melting pointtemperature of the plurality of capacitor-to-die interconnects.
 3. Themethod of claim 1, wherein: the capacitor metallic feature comprisestin; and the die metallic feature comprises gold.
 4. The method of claim1, wherein: the capacitor metallic feature comprises tin and silver; andthe die metallic feature comprises tin and silver.
 5. The method ofclaim 1, wherein: the capacitor metallic feature comprises copper; andthe die metallic feature comprises copper.
 6. The method of claim 1,wherein the step of attaching the capacitor to the flip chip die isperformed while the flip chip die constitutes part of a larger intactwafer comprising a plurality of flip chip dies including the flip chipdie; further comprising dicing the wafer containing the flip chip die tocreate individual flip chip dies, including the flip chip die.
 7. Themethod of claim 1, wherein the step of attaching the flip chip die tothe substrate comprises reflowing a plurality of solder bumps.
 8. Themethod of claim 1, wherein the plurality of die-to-substrateinterconnects comprise SnBi-based, In-based, and/or PbSn-based solder.